Pigments are finely divided particulate solids which are typically either dispersed in vehicles or coated on a substrate. Pigments may be organic or inorganic, and are usually unaffected by the vehicle or medium in which they are incorporated. They may alter appearance by selective absorption or by scattering light, and are often used as an integral part of decorative, protective and functional coatings. In general pigments are insoluble, unlike dyes, which are soluble.
Pigment particles may vary in size, shape and surface area. In commercial applications, typical pigment particles are from 0.01-10.0 .mu.m in diameter and from 1-1100 m.sup.2 /g in surface area. To achieve complete dispersion in a vehicle, the surface of the pigment particles must be properly wetted.
Extender pigments are colorless or white and generally have a refractive index of less than 1.7. One example of an extender pigment is kaolin, a hydrous aluminosilicate mineral that is used as a filler in the paper and paperboard industry. Calcined kaolins are white and hard, and are used for example in paper and in water based paints. Additional extender pigments include other clays, calcium carbonates, talc (magnesium silicates), and titanium dioxide.
U.S. Pat. No. 5,116,418, hereby incorporated by reference, describes a type of pigment known as a "structured aggregate pigment". Such pigments can include aggregates of particulate matter treated with a "functional microgel component". One commercially sold structured aggregate pigment of this type is Norplex.RTM. 604, made by Nord Kaolin Company of Jeffersonville, Ga.
U.S. Pat. No. 5,336,311, hereby incorporated by reference, describes cationizing a structured aggregate pigment by adding a sufficient amount of a modified or unmodified condensation product of epichlorohydrin and dimethylamine. That patent discloses, for example, that a structured aggregate pigment, such as Norplex.RTM. 604, can be cationized with a suitable modified or unmodified condensation product of epichlorohydrin and dimethylamine. The commercially available pigment Norplex.RTM. 650 (Nord Kaolin) is made in this manner. The cationized structured aggregate pigment of U.S. Pat. No. 5,336,311 is disclosed to exhibit superior retention characteristics when used in papermaking.
Pigments made using cationic polymers are also described in U.S. Pat. Nos. 4,874,466; 5,169,441; 5,006,574; 5,152,835, 4,738,726, and 5,078,793.
Cationic pigments that are to be used for papermaking should have several properties if they are to be most useful. They should be efficiently transportable, particularly by railway car. Specifically, upon addition of cationic polymer the pigments should be capable of forming a reasonably high solids slurry. The pigments of U.S. Pat. No. 5,336,311 are said to be capable of forming a slurry of "a maximum of about 55% solids". This allows relatively efficient transport in a railway tank car. It would be advantageous, however, to form an even higher solids slurry to increase the efficiency of transport (i.e., to transport more pigment and less water). It would also be advantageous to use less cationizing agent to form the high solids slurry.
Furthermore, the pigments should advantageously form a gel when the cationic polymer is added to prevent settling of the pigment during transport. Such settling can make removal of the pigment from the railway car very difficult. It is preferred, however, that the gel not be too hard. The gel should be capable of forming a free flowing liquid when gently agitated, such as with a gentle air stream ("air sparging"). This allows the pigment to be drawn through lines attached to railway car.
In addition to these handling properties, the pigment should be sufficiently retained in a "filled" paper sheet in the papermaking process. Pigment that is not retained enters the waste stream of the papermaking plant, where it must either be recycled or properly treated as a pollutant. The structured aggregate pigment containing a functional microgel of U.S. Pat. No. 5,336,311 has especially advantageous retention characteristics.
There is a need for a pigment that exhibits improved handling characteristics and superior retention characteristics, but which does not employ a functional microgel component as in the structured aggregate pigments described above. There is also a need for a pigment that could, if desired, be formulated at a higher solids level for more efficient transport. In addition, there is a need for a pigment that requires less cationic polymer to make an adequately cationized slurry.